Process for modifying curie temperature of ferromagnetic lanthanide chalcogen solid solutions compounds



Feb. 27. 1968 F. HOLTZBERG ETAL 3,371,041

PROCESS FOR MODIFYING CURIE TEMPERATURE OF FERROMAGNETIC LANTHANIDE CHALCOGEN SOLID SOLUTIONS COMPOUNDS Filed June 11, 1964 2 Sheets-Sheet l y FIG 1 x +30- o co 2 20 a gg +10 in 0. 2 2 I I a I E 2 4 .6 8 1o 35 -10 MOL FRACTION X OF Lose IN 0 EUSB i- L0XSe LuSe F FIG.2 +40 X 3, +30 i 3 +20 {5 2% E 0 0 2 p. LU g -50 E g CL MOL FRACTION x OF GdSe IN EU1 X Gd x Se EuSev GdSe INVENTORS FREDERIC HOLTZBERG THOMAS R.MCGUIRE SIEGFRIED IMETHFESSEL W as I I ATTQRNEY United States Patent York Filed June 11, 1964, Ser. No. 374,351 16 Claims. (Cl. 252-6251) This invention relates to a process for changing the ferromagnetic Curie temperature of semiconducting mag netic compounds of divalent or trivalent lanthanides by changing their electrical conductivity by chemical means such as alloying or formation of solid solutions. These chemical procedures are carried out in such a way that the electrical conductivity is varied by changing the population of electrons in the energy levels of the rare earth compound involving the 5d and/ or 6s energy levels of the lanthanide ions. The variation of electrical conductivity due to the presence of the electrons in the 5d and/or 6s energy levels of the lanthanide ion is necessary as a condition for the above-mentioned Curie temperature change resulting from the process of the invention. Variation in the value of the electrical conductivity in the 5d and/ or 6s energy level allows optimization and/or maximization of the value of the ferromagnetic Curie temperature in these compounds for their specific applications.

The lanthanides or rare earth metals hereafter referred to as (Ln) comprise elements selected from the group having atomic number 57-71 (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) including the elements having atomic numbers 39 and 21 (Y and So).

The process of the invention can be illustrated by its application to the following semiconducting compounds which include divalent and/ or trivalent lanthanide ions:

(a) the 1:1 europium chalcogenides EuO, EuS, EuSe and EuTe having a face centered cubic crystal structure similar to rock salt are selected as representative of the divalent lanthanide semiconductors; and

(b) the gadolinium sesquichalcogenides (e.g.,

etc.) have the body centered cubic Th l structure and for small values of z they are examples of trivalent lanthanide semiconductors. The electrical resistivity of typical samples of these compounds measured at room temperature is set forth in Table I as PRT.

These compounds are ferromagnetic or antiferromag netic below the critical temperature T (Curie temperature) or T (Nel temperature), respectively, which, together with the paramagnetic Curie temperature 0, are set forth in Table I. The paramagnetic Curie temperature 0 is defined by the Curie-Weiss law x as the temperature T for which the magnetic susceptibility 3,371,041 Patented Feb. 27, 1968 goes to infinity where I is its magnetic moment per cm. of material and H the applied magnetic field. C is a constant derived from the atomic magnetic moments of the lanthanides given in Table II. According to molecular field theory, the paramagnetic Curie temperature 6 is related to the interaction J between each atomic spin moments S (see Table II) and its neighboring Z atoms by the expression (8' 1) S 0=2/3 ZiIZ J (k=Boltzman constant), (summed over i atoms). Therefore, the value of the paramagnetic Curie temperature 6 is herein used as a measure of the magnetic interaction. Positive values of 0 indicate positive interaction resulting in ferromagnetism below a critical temperature T (ferromagnetic Curie temperature). Negative values of 6 result in antiferromagnetic properties below a critical temperature T (Nel temperature). In the ferromagnetic state (T T the material has a spontaneous magnetization. The spontaneous magnetization 411-1 approaches the saturation value of 411-1, per cm. of material (Table I) for T=0 K. when a strong magnetic field H is applied. The

interaction between atomic magnetic moments which produces the spontaneous magnetization in the ferromagnetic state at temperatures below the Curie temperature T has been explained for the Eu-chalcogenides by an interaction between the spins of the nearest-neighbor Eu ions in the rock salt-type crystal structure. The antiferromagnetic interaction, however, is between next nearestneighbors via electrons localized around the anions (i.e., magnetic superexchange). (T. R. McGuire, B. E. Argyle,

M. W. Shafer, J. S. Smart, Magnetic Properties of Some Divalent Europium Compounds, Journal of Applied Physics, 34, 1345-46, 1963).

The ferromagnetic saturation moments of EuO, EuS, and EuSe, as given in Table I, are higher than those of ferrites (with saturation moments below 6000 gauss). Therefore, the semiconducting Eu chalcogenides provide a useful magnetic material for applications where high electrical resistivity and high magnetic moment is desired (high frequency transformer, memory cores, etc.). The low Curie temperatures of the Eu chalcogenides, on the other hand, restrict their use to temperatures below that of liquid N Procedures providing an increase of the Curie temperature of these compounds, therefore, increase the range of their technical applicability in magnetic devices. Since the magnetic permeability ,u.

i... up Bas n for low values of the applied magnetic field H has a maximum at the Curie temperature, it is advantageous in device applications (e.g., magnetic transformers) to adjust the Curie temperature of rare earth chalcogeniides to a value close to the Working temperature by using the process of the invention. Materials with a predetermined Curie temperature are also used as cores and temperature sensitive elements in switching devices for control and safety applications.

The compounds Gd S and Gd Se are other examples of semiconducting compounds comprising trivalent lanthanides. They are antiferromagnetic below the Nel temperatures T listed in Table I. (F. l-Ioltzberg, T. R. McGuire, S. Methfcssel, and J. C. Suits, Ferromagnetisnr in Rare Earth-Group V and VI Compounds with Th P Structures, Journal of Applied Physics, 35, 1033- 1040, 1964.)

The chemical bonding in lanthanide semiconductors can be explained as arising principally from electrostatic coulomb forces (polar semiconductors) resulting from the transfer of two or three valence electrons from the divalent or trivalent lanthanide atoms into the p electron shells of the O, S, Se, or Te atoms, respectively. This electron transfer fills the p-shell to the noble gas configuration of Ne, Ar, Kr, or Xe, respectively, providing the anions with twofold negative charge. Consequently, the lanthanide ions have, in semiconducting compounds, the electron configuration of the noble gas atom xenon, but the 4f shell is partially filled and is responsible for the localized magnetic moment of these compounds.

The following atomic magnetic moments and atomic spin moments S (given in Bohr magnetron units ,u are assigned to the lanthanide ions of a given chemical valency:

TABLE II Lanthanide Valence Spin Moment S Magnetic Moment (SJ (plus) 1/2 (#13) Bohr magnetons) The magnetic interaction between the atomic magnetic moments of lanthanide ions in semiconductors, which produces their ferromagnetic or antiferromagnetic alignment at temperatures below the Curie temperature, T or Nel temperature, T respectively, has been investigated for the Eu-chalcogenides EuO, EuS, EuSe, and EuTe. The ferromagnetic or antiferromagnetic interaction, which has been discussed phenomenologically considering first and second-nearest neighbor interaction (T. R. McGuire, B. E. Argyle, M. W. Shafer, and I. S. Smart, Magnetic Properties of Some Divalent Europium Compounds, Journal of Applied Physics, 34, 1345-46, 1963), has not been related to the low electrical conductivity in these compounds. The observed decrease of 0 in the compound series EuO, EuS, EuSe, and EuTe is correlated with change in the Eu-Eu distance corresponding to the increasing lattice constant of the rock salt type crystal structure.

Several 1:1 chalcogenides of trivalent lanthanides (e.g., GdS, GdSe, LaS, LaSe, YS, YSe, etc.) are also known to crystallize with the NaCl-type structure. These cornpounds have metallic-type electrical conductivity of about 10 (ohm cm.) at room temperature resulting from one metallic electron per formula unit in the 1: 1 rare earth compound of the trivalent lanthanides with divalent chalcogens. This excess electron is assume-d to be in energy levels related to the 5d and/or 6s subshell of the trivalent lanthanide ions. The metallic compounds GdS and GdSe, as examples, are antiferromagnetic materials, but no reliable magnetic measurements have thus far been reported. There is uncertainty about basic properties, such as crystal structure, of GdTe and the existence of the compound GdO is not known. semiconducting compounds of trivalent lanthanides with elements of Group VI of the periodic table occur as the sesqui-oxides, -selenides, -sulfides, and -tellurides (Ln S Ln Se Ln Te at the composition 2:3, with a certain solubility of the metal ions, described by a formula of the type Ln S Ln Se Ln Te where 0 z5c and c is a constant for each solid solution system O c5l, thereby limiting the concentration of the formation of the thorium phosphide type crystal structure in the phase equilibrium diagram. Experimentally the maximum value of z =c depends on the system and for Gd Se is C=5/8 for example.

Theories (P. G. DeGennes, Compt. rend., 247, 1836, (1959)) interpret the magnetic interaction between the atomic magnetic moments in lanthanide metals as taking place via conduction electrons (indirect exchange: I.E.). Calculation of the paramagnetic Curie temperature 0 using the LE. theory for cubic crystal lattices in the molecular field-approximation has been worked out (D. Mattis, N. Anthony, and L. Horwitz, IBM Report RC945 (1963)) and shows that interaction by LE. for small concentrations of free electrons in the conduction band can be expected to be ferromagnetic. The applicability of the LE. theory to metallic lanthanide compounds has not been investigated thoroughly and is apparently not valid for semiconducting compounds.

It is an object of the invention to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by adjusting the electrical conductivity.

Another object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by adjusting electrical conductivity by changing the chemical composition of the rare earth semiconductor.

A further object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by adjusting the electrical conductivity by partially replacing ions in the rare earth semiconductor with ions of different valence, or by changing the anion to cation ratio by a prescribed amount.

Still a further object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by changing the electron population in the 5d and/or 6s electron energy levels of the lanthanide ions in the rare earth semiconductors.

Further, still another object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by changing the electron population in the 5d and/ or 6s electron energy levels of the lanthanide ions in the rare earth semiconductors by changing the chemical composition of the rare earth semiconductor.

Still another object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in rare earth semiconductors by changing the electron population in the 5 d and/ or 6s electron energy levels of the lanthanide ions in the rare earth semiconductors by partially replacing ions in the rare earth semiconductor with ions of different valence or by changing the anion to cation ratio by a prescribed amount.

Further, another object of the invention is to provide a process for changing the magnetic Curie temperature or to introduce ferromagnetism into Eu chalcogen 1:1 compounds or Gd or Tb chalcogen 2:3 compounds (excluding Gd O Tb O resulting in a predetermined value of the Curie temperature by adjusting the electrical conductivity.

A further object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism into Eu chalogen 1:1 compounds or Gd or Tb chalcogen (2+2) :(3+z) compounds (excluding Gd O Tb O by adjusting the electrical conductivity by partially replacing ions in the rare earth semiconductor with ions of different valence or by changing the anion to cation ratio by a prescribed amount.

Still a further object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism into Eu chalcogen 1:1 compounds or Gd or Tb chalcogen (2+z):(3+z) compounds (excluding Gd O Tb O by changing the electron population in the 5d and/ or 6s electron energy levels of the lanthanide ions in the rare earth semiconductors.

Further, still another object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in Eu chalcogen 1:1 compounds or Gd or Tb chalcogen (2+z):(3+z) compounds (excluding Gdzog, TB O by changing the electron population in the 5d and/ or 6s electron energy levels of the lanthanide ions in the rare earth semiconductor by changing the chemical composition of the rare earth semiconductor.

Still another object of the invention is to provide a process for changing the magnetic Curie temperature to a predetermined value or to introduce ferromagnetism in Eu chalcogen 1:1 compounds or Gd or Tb chalcogen (2+z):(3|-z) compounds (excluding Gd O Tb O by changing the electron population in the 5d and/ or 6s electron energy levels of the lanthanide ions in the rare earth semiconductors by partially replacing ions in the rare earth semiconductor with ions of different valence or by changing the anion to cation ratio in a prescribed amount.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

The invention describes a process of introducing ferromagnetism in paraor antiferromagnetic rare earth compounds having semiconducting electrical properties (e.g., EuTe, Gd t se i Gd S etc. for small values of z), or to change the ferromagnetic Curie temperature T (i.e., the temperature below which the atomic magnetic moments are aligned to produce spontaneous magnetization) in semiconducting ferromagnetic compounds (e.g., EuO, EuS, EuSe, Gd S Gd Se etc., for larger values of 2).

In order to obtain this variation of the magnetic properties of the lanthanide semiconductors, the process of the invention varies the electrical conductivity of these materials. The desired variation in electrical conducivity is obtained by the alloying or by formation of solid solutions in one of the following three ways:

(1) The lanthanide ions in the semiconductor are partially replaced by metal ions which have, in the chemical reaction forming the semiconducting compound, a different positive valence than the lanthanide ions in the original compound. Examples of this procedure are the formation of solid solutions between the ferromagnetic semiconducting compounds EuO, EuS, EuSe, and the metallic compounds GdS, GdSe, LaO, LaS, LaSe, YS, YSe.

The variation of paramagnetic Curie temperature with LaSe concentration x in the solid solution system Eu La Se is shown in FIG. 1, as a specific illustration (where Ogxsl). In this case, the addition of the metallic nonmagnetic compound LaSe to the ferromagnetic semiconductor EuSe results in an increase in Curie temperature to a maximum between and mole percent LaSe and in a decrease with further increase of the LaSe concentration. Simultaneously the electrical conductivity increases with increased LaSe concentration.

FIG. 2 shows, as an example, the variation of 0 in the solid solution system Eu Gd Se(0gxg1) between a ferromagnetic semiconductor (EuSe) and an antiferromagnetic metallic compound (GdSe). In this case, the paramagnetic Curie temperature rises sharply with the GdSe concentration x to a maximum between 0.15 and 0.25 mole fraction of GdSe and then decreases, becoming negative for larger concentrations of conduction electrons in the system. Negative 0 values indicate the presence of antiferromagnetic interactions between the atomic magnetic moments of the lanthanide ions in the compound.

(2) The anions (e.g., O, S, Se, Te) in the lanthanide semiconductor are partially replaced by chemical elements which show, in the chemical reaction forming the semiconducting compound, a different negative valence than the anions in the original compound. An example of this procedure is the formation of a solid solution between the rock salt compounds GdS, GdSe and GdAs. The compound GdSe, as a more specific example, is antiferrornagnetic with a 6 value of -60 K. If one third of the divalent selenium anions are replaced by trivalent As anions the 0 value changes to +6 K.

3. The ratio of the number of cations-to-anions per molecule is made different from that of the original semiconductor. Examples of this procedure are the formation of solid solutions between Gd S or Gd Se and Gd, Y, Nd, Fe, Al, etc., in order to change the mag netic properties of Gdgsg or Gd Se from antiferromagnetic to ferromagnetic. FIG. 3 shows the variations of the paramagnetic Curie temperature 6 and of the electrical conductivity at room temperature of the antiferromagnetic semiconductor Gd Se with increasing Gd concentration y in excess of the concentrations 2:3. The excess parameter y is related to the concentration parameter z in the following way: 1 mole of Gd Se +z moles of GdSe result in 1 mole of Gd Se 1 which is identical with l 2y moles of Gd Se for For an excess of more than y=0.04 Gd atom per molecule, the paramagnetic Curie temperature 0 changes from negative to positive values, indicating the transition from antiferromagnetic to ferromagnetic properties.

FIG. 4 relates the paramagnetic Curie temperature 9 of the compound Gd Se to the value of electrical conductivity measured at room temperature. The variation of the electrical conductivity is an indicator of the varying electron population of the 50? and/or 6s energy levels forming the conduction band. FIG. 4 also includes, as another example, the increased 3 value of the compound Y Gd Se obtained by adding YSe t0 Gdzseg.

Normally, alloying or formation of solid solution of a ferromagnetic lanthanide semiconductor with nonferromagnetic substances (e.g., EuS with SrS) decreases the ferromagnetic transition and Curie temperature. However, a characteristic of the alloying or formation of solid solutions between rare earth semiconductors and nonferromagnetic materials used in the process of the invention is that it can also increase the ferromagnetic Curie temperature, or can generate ferromagnetic properties or spontaneous magnetization in paramagnetic or antiferromagnetic lanthanide semiconductors. This variation in magnetic properties is related to and controlled by the value of the electrical conductivity.

A more specific form of the process of the invention can be derived by the following considerations. In the indirect exchange mechanism the conduction electrons become polarized with regard to their magnetic spins by an exchange interaction with the uncompensated spins in the partially filled 4 energy levels of the lanthanide ions. This exchange interaction is mainly effective for conduction electrons in certain energy levels, namely, electrons in the 5d and/or 6s levels of the lanth-anide ions or in corresponding levels in lanthanide alloys and compounds. The indirect exchange coupling via the spin polarization of the conduction electrons yields the ferromagnetic alignment of the magnetic moments of the lanthanide ions below the ferromagnetic Curie temperature. The indirect exchange can be most effective via electrons in the 5d and/or 6s energy levels of those lanthanide ions which carry the magnetic moments to be aligned. The purpose of the process of the invention is to introduce ferromagnetic alignment or to change its strength by introducing or changing the indirect exchange coupling in lanthanide semiconductors by a variation in the number of conduction electrons. Therefore, the process of the invention in its particularly effective form describes the alloying or solid solution formation of paramagnetic, ferromagnetic, or antiferromagnetic lanthanide semiconductors with elements or compounds in such a way that the population of electrons in the d and/ or 6s energy levels of the lanthanide ions is changed in order to affect the magnetic interaction produced by I.E. The variation of electrical resistivity observed in such a case can be used as an indicator of the induced or changed electron population in the 5d and/or 6s energy levels of the lanthanide ions.

On the other hand, in several chemical reactions variations of the electrical resistivity in lanthanide semiconductors can be observed which are not related to the variation of the electron population in the 5d and/or 6s energy levels of the lanthanide ions and therefore not effective in varying the magnetic properties. An example is the increase of electrical conductivity due to tantalum impurities which does not influence the paramagnetic Curie temperature. It is characteristic for the process of the invention that the electron population in energy levels suitable for indirect exchange coupling is changed by chemical reaction. Suitable materials for changing the electron population in the 5d and/or 6s energy levels in lanthanide semiconductors must be selected by examination of spectroscopic and chemical data for the elements. The electron population in the 5d and/or 6s level of EuSe, as an example, can be changed by partially replacing the Eu ions by other metal ions. The condition for increasing the electron population in the conduction band of EuSe is that the replacing metal ions have electrons in an energy level in the semiconducting compound higher than or equal to the energy of the 5d and/or 6s energy level of the Eu ions (donor type impurities). The position of this 50? energy level is given by the forbidden energy gap of the semiconducting lanthanide compound (e.g., 1.85 ev. for EuSe; 2.21 ev. for Gd Se The ferromagnetic Curie temperature related to indirect exchange interaction increases with increasing electron population in the 5d and/or 6s energy level, and at certain values of the electron population it reaches a maximum and then decreases again (e.g., FIGS. 1 and 2). Therefore, it is possible to optimize or maximize the ferromagnetic transition temperature of lanthanide semiconductors by the process of the invention according to desired device applications.

The electrical conductivity produced by the process of the invention can be so large in certain cases that the material is no longer considered to be a semiconductor, which should not be considered to be contrary to the meaning of the patent.

Example 1 For a specific device application a semiconducting ferromagnetic lanthanide compound with a paramagnetic Curie temperature of 9=30 K. may be required. As seen from FIG. 1, the 0 value of EuSe increases from 9 K. to 30 K. by adding LaSe corresponding to a number of 0.25 electrons per (Eu La Se) molecule. The same 0 value of K. can be obtained by adding 1.5 mole percent GdSe to EuSe (see FIG. 2) with a smaller number of conduction electrons, i.e. higher electrical resistivity. Since the Gd ions have a magnetic moment of 7,1 per atom (Table II) the solid solution of EuSe with GdSe does not change the ferromagnetic saturation moment per molecule. However, the addition of LaSe lowers the magnetic saturation moment proportional to the La concentration. Since the slopes of the curves in FIGS. 1 and 2 are different, the compounds Eu Gd Se has a higher sensitivity of 0 to concentration than the compound Eu La Se. Materials of the composition described above can be prepared in the following way:

1(a). Eu La Se: 1.7319 grams of EuSe, finely powdered in a dry, oxygen-free atmosphere, are mixed with 0.5447 gram of finely powdered LaSe and pressed (in a He atmosphere) into pellets which are placed in a tantalum crucible which is evacuated and vacuum sealed by cold welding. The tantalum crucible is placed on a pedestal in a quartz vacuum system centered in a radio frequency induction heating coil. An ambient atmosphere of dry helium or other inert gas may be used instead of vacuum. Power is delivered to the coil at a rate such that the crucible temperature rises to the melting point of the solid solution system, 2000 C.2500 C., as read with an optical pyrometer. The temperature is held at this level for 10-20 minutes to insure homogeneity and then the power to the coil is turned off for rapid cooling. The Eu La Se appears as a dense, brittle ingot. The material has a positive paramagnetic 0 value of +30 K. Materials of other 0 values and compositions can be obtained by corresponding changes in the weight ratio of EuSe to LaS.

1(b). Eu Gd Se: The procedure of Example 1-a is repeated except that 2.2746 grams of EuSe is mixed with 0.0354 gram of GdSe. The compound obtained by this procedure has a positive 0 value of +30 K. Materials with other 0 values and composition can be obtained by corresponding changes in the weight ratio of EuSe and GdSe.

In a modification of the process, the material is not melted but sintered by heating to a temperature below the melting point during a time long enough to provide a homogeneous product.

Example 2 In a specific device application, a ferromagnetic lanthanide compound with a Curie temperature of 45- -1 K. may be required. FIG. 4 shows that the antiferromagnetic compound Gd Se can be made ferromagnetic with a Curie temperature adapted to the desired value by lowering the electric resistivity to 0.01 ohm-cm. at room temperature. This value of electrical resistivity must be obtained by adding another component to Gd Se or partially replacing Gd ions in such a Way that the population of electrons in the 5d and/ or 6s energy level of the Gd ions is increased. From chemical and spectroscopic data, one can expect, that an excess of trivalent elements of the group of the lanthanides or yttrium should fulfill this requirement due to their chemical similarity to gadolinium. The following part of the process of the invention is demonstrated for two specific examples. In the first one (2(a)), the electrical resistivity is decreased by adding Gd metal in excess to the composition 2:3. From FIG. 3, one reads the composition Gd Se corresponding to 0=45 K. In the second example (2(b)), the desired change in electrical resistivity is obtained, in a more general way, by adding Y and partially replacing Gd ions by Y ions, corresponding to a final composition of Y Gd Se 2(a). Gdz gseg gzi 4.6315 grams of Gd Se finely powdered in a dry, oxygen-free atmosphere are mixed with 0.9448 gram of GdSe treated in the same way. The mixture is pressed (in an inert atmosphere) into pellets which are placed in a carbonized tantalum container, graphite container, or any other container made of material which does not enter into the reaction. The size of the pellet is such that the pellet provides a piston fit to the crucible. A tapered plug of crucible material is forced into the crucible so that it presses on the surface of the uppermost pellet in order to exclude as much dead (i.e., empty) volume as possible and provides a pressure tight seal of the container. The tight fit is necessary because if there is dead (or empty) space in the crucible, the Se will vaporize and condense out on cooling and an inhomogeneous pro-duct will result. The crucible is then placed on a pedestal of refractory material in a quartz 10 6s energy levels of the lanthanide ions in the semiconducting compounds.

While the invention has been particularly shown and described with reference to preferred embodiments therevacuum system centered in a radio frequency induction of, it will be understood by those skilled in the art that heating coil. An ambient atmosphere of dry helium or the foregoing and other changes in form and details may another inert gas is often used instead of the vacuum. be made therein without departing from the spirit and The container is heated to approximately 1700 C. at a scope of the invention. rate of about l00/min. The temperature, monitored by What is claimed is: a pyrometer, is then raised to the melting point of the 1. A solid solution system having the formula compound. After heating for 10 minutes at this temperature, the power is turned oh and the sample cooled to Eu R,,O room temperalgurfi. '1"ll1)e 61412118 29 (1 0 m; f (where 0.015 x 1 and R is selected from the group denseningot W 1c is r1tt e and 0x1 1zes s ow y n I'IOlSt consisting of Nd, La, Y, Ce and air. e room temperature e ectrical res1st1v1ty 1s a on The Solid solution EUO'QNdOJQ 0.01 ohm-cm. and the material shows ferromagnetlc be- 3 The solid Solution Eu La 0 havior corresponding to a ositive 0 value of 45 K P 4. The solld solution Eu Y O. In a mod1ficat1on of the process, the matenal is not 5 The Solid solution Eu C O 0.8 0.2 melted but smtered by heating to a temperature below 6 The Solid Solution Eu Sc 0 h lting point during a time long enough to provide M t Z d t 7. A process for modifying magnetic Curle temperaa fi f? 1 b M d b ture of semiconducting divalent lanthanide rare earth 39 S 5 2;? ,53? :1 9 5 t &3? chalcogenides to a desired value in which the divalent 5 g? g m e Wale m 10 2 e8 0 lanthanide rare earth constituent of said divalent lantha- Tgl 05d 1.6 f z];- 1 2 d tth t nide rare earth chalcogenide is partially replaced by a e Proce we 0 Xampe 1S repeate excel) a trivalent lanthanide rare earth element comprising the 4.4110 grams Gd Se are mlxed with 0.8393 gram of Steps of: YSe. The resulting material has ferromagnetic properties 1 t rm 1 t h 1 corresponding to a positive 0 value of 45 K. Materials a g i nva g. th e i i 5 3 With other 0 values can be obtained by corresponding a g 23 i i change in the composition and in the weight ratio of ear eemen o c acoPem e is W1 Sal Gd S divalent rare earth chalcogenide; and

2 c and YSe.

, (b) heatlng the thus formed mixture to the melting Examples 3-26 point temperature of sand mlxture and coollng said For Examples 3-26 the process of Example 1(a) is mixture to produce a homogeneous solid solution repeated except that the constituents and the amounts as therefmmlisted in Table III are used. The products formed and the 8. A process for modifying the magnetic Curie tem- 0 values are also set forth, together with their reaction perature of a semiconducting europium chalcogenide havtemperatures, in Table III. ing the formula EuA (where A is at least one element TABLE III Example N o. Amt, Constituent Amt, Constituent Product; 0 KJ p- 2.3217 gr., GdAs--- Gd, ASO.5SGO.5 4 2,000 2.3092 gr., EuSe Gd0.5El10,5S8 +20 2, 000 1.3895 gr., E118" E110 .15Y0 1589025 1175 900 1.3801 gr., EuS EUt],75 d0.25 0.25 0.75 000 1.4983 gr., GdS.. GdunsSroasS -35 2,000 0217:; gr., LaSe- LZ OJ UOJl 2,000 2.3763 gr., EuTe 0.4272 gr., G-dTe Eu0,s5Gd0 15Te +5 2,000 0.5591 gr., Eulo 1.8474 g1., EuSe EuTemSe +10 2,000 1.8896gr.,GdSe..- 0.2039gr.,NaSe Gd0.aNao 1Se 0 1,400 0.2378 gr., TbSe 2.0783 gr., EuSe TbmEunsSe +35 2,000 0.2414 gr., DySe...v 2.0783 gr., EuSe Dyu.1Euo,0Se +40 2, 000 0.2539 gr., LuSe 2.0783 gr., EuSe. LL10.1Eu0.0Sc +17 2,000 0.2438 gr., HoSe 2.0783 gr.,EuSe EuMHOMSe 000 0.2462 gr., ErSe 2.0783 gr., EuSe- EumErmSe 2,090 0.4957 gr., TrnSe... 1.8474 gr., EuSe Ell0.sTII10.2SB +25 2,000 2.1937gr.,EuSe 0.1260gr.,YbSe. Eu0,05Yb0.o5Se +15 2,000 1.9628 gr., EuSe- 0.1858 gr., ScSe Euwscmse +20 2,000 2.2853 gr., LuSe..- 0.2438 gr., HoSe-.-. Luo.0Ho0.1Se +20 2, 000 0.0774 gr., L210 1.58 31110551190050 y 000 0.3120 gr., 060.. 1.34 EuMOeMO +85 2,000 0.1602 gr., NdO 1. E110 .nNdoJO +85 2,000 0.1570 gr., YO 1.4285 gr., Eu0 Euo t5Yo.15O +85 2,000 0.0009 gr., ScO 1.511s gr., Euo. EUMSCOJO +85 2,000 0.5097gr.,NaSe 1.1810 gr., Grass..." GdMNaMSe +12 2,000

The process of Example In is repeated except that 1.18105 grams of GdSe and 0.02268 gram of Gd are used. The 0 value is K.

The solid solution system having the formula (Where 0 x l and R is selected from the group consisting of Nd, La, Y, Ce and Sc) represents a group of new eruropium oxide solid solutions (Examples 21-25).

Thus, the invention describes a process for adjusting the ferromagnetic Curie temperature of semiconducting magnetic lanthanide chalcogenide compounds to a predetermined value by changing their electrical conductivity. The electrical conductivity is controlled by chemical means such as alloying or formation of solid solutions. The process is particularly efiective when the chemical procedure effects the electron population in the 5d and/ or wherein the Eu has been partially replaced to change electrical conductivity and produce a different magnetic Curie temperature than starting material EuA. 9. A process for modifying the magnetic Curie tem- 1 l perature of a semiconducting chalcogenide having the thorium phosphide type crystal structure and having the formula Ln A' (where O x I) which comprises admixing said chalcogenide with an element Ln wherein A is at least one element selected from the group consisting of sulfur, selenium and tellurium; Ln is at least one element selected from the group consisting of lanthanum, cerium, praesodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and scandium, Ln is at least one element selected from the group consisting of lanthanum, cerium, praesodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbuim, thuliurn, ytterbiurn, lutetium, yttrium, scandium, strontium, calcium and barium; and heating said mixture to a temperature of from 1400 C. to 2500 C., to produce a homogeneous solid solution 12 having the general formula (Ln Ln" A' where O y 1 and (l z /s.

10. The process of claim 9 wherein a chalcogenide compound containing Ln is used in place of the element Ln.

References Cited UNITED STATES PATENTS 2/ 1966 Matthias 252-62.5 5/1966 Borchardt 252-301.5

OTHER REFERENCES TGBIAS E. LEVOW, Primary Examiner.

ROBERT D. EDMONDS, Examiner. 

1. A SOLID SOLUTION SYSTEM HAVING THE FORMULA 